Stroke is a leading cause of death, long-term disability, and socioeconomic costs, highlighting the urgent need for more effective treatments. Intravenous administration of tissue plasminogen activator (t-PA) is the only FDA-approved therapy to re-establish cerebral blood flow. But because of increased risk of hemorrhage beyond 3 h post stroke, few stroke patients (1-2%) benefit from t-PA; t-PA, which has neurotoxic effects, can also aggravate the extent of reperfusion injury by increasing blood-brain barrier (BBB) permeability. An alternative strategy is needed to extend the window of intervention, minimize damage from reperfusion injury, and promote brain repair leading to neurological recovery. Reactive oxygen species (ROS), generated soon after ischemia and during reperfusion and thereafter, are considered the main mediators of ischemic injury. Our preliminary studies show that nanoparticle-mediated delivery of the antioxidant enzyme superoxide dismutase (SOD) to the brain mitigates the effects of ROS, protects the BBB, and significantly reduces reperfusion injury in stroke model. Based on these results, we envision that an efficient antioxidant delivery system to the brain (nano-SOD/catalase) in combination with t-PA could overcome the limitations of current therapy with t-PA alone. We hypothesize that nano-SOD/catalase, with its sustained neuroprotective effects and reduced reperfusion injury to the BBB, would 1) minimize damage from t-PA while retaining its thrombolytic activity to extend the window of treatment, 2) inhibit neuroinflammation by neutralizing the deleterious effects of ROS, and 3) create conditions favorable to neuronal repair to regain neurologic functions. The proposal's four specific aims are: AIM 1: To develop a neuroprotective nano-SOD/catalase system. AIM 2: To analyze the neuroprotective efficacy of nano-SOD/catalase against ischemic injury. AIM 3: To evaluate the extent of neurological recovery and motor functions with nano-SOD/catalase and to explore its prophylactic use. AIM 4: To confirm the brain repair mechanisms of nano-SOD/catalase following ischemic injury. An effective treatment could benefit more stroke patients than can be treated with t-PA alone because of the extended window of treatment and reduced risk of hemorrhagic complications. Furthermore, it could significantly improve the post-stroke disability because of neuronal repair.